The present invention relates, in general, to the field of true RMS power demodulation systems and methods for a modulated RF signal. More particularly, the present invention relates to scalar network analyzers and systems and methods for power demodulation of a modulated RF signal for deriving true RMS power and eliminating the errors inherent in conventional equipment.
Certain microwave measurements require a modulation to be placed on the stimulus signal in order to permit the receiver to distinguish the desired signal from any unmodulated interfering signal which may leak into the test system. Typical measurements requiring such a tag to be placed on the desired signal are mixer or antenna measurements. True RMS measurements are especially critical when the signals have significant amounts of harmonics or modulation. This is often the case when measuring active devices running into compression.
Typical systems use a microwave detector and then use the difference between the detected voltage when the modulation is on and the detector voltage when the modulation is off to calculate the appropriate power measurement. Such a technique provides good rejection of the interfering signal in the sense that the detector reads zero when there is no modulated signal, however, the linearity of the detector is severely affected by the presence of the unmodulated signal.
These prior art techniques AC couple the output of the detector to remove the unmodulated portion incident upon the detector. Such a technique works satisfactorily given a linear detector, however diode detectors are inherently non-linear devices and most be corrected for their non-linearity. Conventional correction techniques lead to errors in measurement of the modulated RF power as amplitude of the interfering signal grows.
It would thus be highly desirable to provide a true RMS power demodulation system and method to eliminate the errors which are present in conventional equipment. It would further be highly desirable to provide a true RMS power demodulation system and method for a modulated RF signal which may be used in a scalar analyzer and which offers significantly improved accuracy over conventional scalar analyzers in AC detection modes, especially in the presence of interfering signals.